This invention relates to a process for the preparation of phenylphosphonous dichloride. More particularly, the present invention relates to a continuous high-pressure process whereby phenylphosphonous dichloride is produced in high yields with negligible coproduction of polychlorinated biphenyls.
Phenylphosphonous dichloride is an important industrial intermediate which is used in the manufacture of the insecticide EPN, in the manufacture of nylon stabilizers and in the manufacture of organophosphorus compounds.
Of the several known methods by which phenylphosphonous dichloride can be prepared, the "hot tube" process and the batch "autoclave" process are perhaps the two most prominent.
In accordance with the basic hot tube process, benzene and phosphorous trichloride are vaporized to form a mixed vapor stream which is then caused to come into contact with the surface area of a "hot tube". The temperature of the surface of the hot tube is typically maintained at about 600.degree. C. through the use of external electrical heaters.
An improved hot-tube process, wherein monochlorobenzene is added to the reaction mixture, is taught in U.S. Pat. No. 3,029,282.
The batch autoclave process for preparing phenylphosphonous dichloride is described in U.S. Pat. No. 3,864,394, which also teaches that improved yields can be obtained by maintaining a specified relationship between reaction time, reaction temperature and the ratios of phosphorus trichloride and elemental phosphorous to monochlorobenzene present.
Each of these two processes are characterized by certain disadvantages. For example, the hot tube process is subject to formation of tarry residues (about 20-30 grams residue can be formed for every 100 grams phenylphosphonous dichloride produced), which can lead to fouling of downstream equipment, has a tendency to generate undesirable byproducts such as biphenyl, chlorobenzenes and chlorphenyl phosphorous dichloride and is characterized by a very low conversion rate, which makes it necessary to recycle large amounts of unreacted raw materials.
The batch autoclave process, on the other hand, represents a substantial technical advance over the hot tube process, but is still less than satisfactory. The batch process requires a large inventory of reactive materials in the reactor, which can be of concern. In addition, the repeated heating and cooling associated with the batch cycles can have an adverse effect on the life of the reactor.
A process which incorporates the advantages of the batch autoclave process without the disadvantages inherent in the batch nature of the process would represent a welcome advance of the state of the art.